1. Field of the Invention
The present invention relates to printing images on media, such as webs of paper, that will be subjected to pressure during inking and/or folding processes.
2. State of the Art
Conventional printing presses print images on media, such as webs, ribbons or signatures, and then fold the printed media. However, folders and rollers used to grip the media and/or set the folds can damage printed images located at and near the folds. This is because the pressure generated on the media by the folders and rollers can cause the ink on the media to rub off onto a folder or roller, and can cause the ink to smear. In addition, the folders and rollers often press an image side of a first print medium against a second print medium, as for example when multiple sheets or ribbons of print media are folded together to form a broadsheet, tabloid, magazine or digest product. The resultant pressure can cause the image on the first print medium to be "set off", or transferred in whole or in part, to the second print medium. This damages the image on the first print medium and any image at the corresponding location on the second print medium.
When multiple colors are to be printed on a print medium, the print medium is sequentially processed by different print units within the printing press. Each print unit uses a different color ink, and supplies the ink or makes the ink available across the entire width of the print medium, for use as necessary in the image to be printed. Some presses are configured with sequential print units that each provide the same color ink (black for example). Sequential print units are typically used to print the same color ink when one of the print units will print black text and the other print unit will print black images or image components. Even in this situation, each of the print units supplies ink or makes ink available across the entire width of the print medium for use as necessary in the image (text or graphic) to be printed.
FIGS. 2-4 show a front view of a folder unit 200 of a conventional web fed printing press. As shown in FIG. 2, a web of print medium 212, such as paper or any other suitable print medium, flows over a former board 202 in the direction 216. The sides of the print medium 212 form corners 214 as they pass over the edges of the former board 202, and a fold or crease is formed at the nose 213 of the former board 202. The sides of the print medium 212 are rotated 90 degrees in the folding process, so that the line 215 represents the folded edge of the print medium 212 as the folded print medium passes through the nips formed by the rollers 204, 206, 208 and 210. The rollers 210 are cutting cylinders for cutting the folded print medium 212 crosswise. Multiple webs, or ribbons of print media cut therefrom, can be simultaneously processed using the folder unit 200, by pulling the webs or ribbons over the former board 202 in multiple layers.
In at least one of the roller sets 206 and 208, each roller is knurled where it contacts the folded edge 215 and the outer edges of print medium 212. The knurling allows the rollers 206, 208 to grip the print medium 212 and pull it over the former board 202 and through the first set of rollers 204.
FIG. 3A shows a side view of the folder unit 200, with the print medium 212. FIG. 3B shows a top view of rollers 306 which can be used in place of either the rollers 206 or 208. Each roller 306 has a center shaft 308, and ends that have a larger diameter than that of the shaft 308. The surfaces 307 of the roller ends are knurled or textured. FIG. 3B also shows how two layered ribbons 312, 313 of print medium that were pulled over the former board 202 are squeezed together at their folded and outer edges. Generally, the more layers of ribbons there are, the greater the pressure the nip must apply to grip and pull the ribbons over the former board 202 and through the nip without having any of the inner layers of ribbons slip and move with respect to the other layers of ribbons.
FIG. 4 shows a side view of another example of a conventional folder unit. The folder unit 400 has two former boards 202 for simultaneously processing two or more two streams of print media, one stream over each former board 202. The nips formed by the rollers 406, 408 and 410 can simultaneously receive both print streams, and are appropriately knurled to grip the folded and outer edges of each print stream.
Images printed on the outer surface of the FIG. 3 print medium 212 or the FIG. 4 print medium 412 can be damaged by rubbing off onto the rollers, and the rubbed off ink can damage subsequent images by later transferring from the rollers onto a subsequent portion of the print medium. In addition, referring to FIG. 3A, images printed on the inner surface of the print medium 312 can be set off onto the outer surface of the print medium 313, thus damaging images on both the inner surface of the print medium 312 and the outer surface of the print medium 313. Images on opposing faces of the inner surface of the print medium 313 can also be set off onto each other or smeared by pressure from the knurled portions 307 of the rollers 306. Pressure exerted by the nose of the former board 202 on webs or ribbons of print media passing over the former board 202 can also damage images on the print media in similar ways.
Some commercially available inks (e.g., the so-called "high performance" inks known to the industry) are resistant to transfer or smearing by pressure and abrasion. These inks have been used to replace lower quality inks in printing units where high quality printing is desired. Although using such inks can reduce or eliminate image damage caused by pressure, such inks are typically expensive. Accordingly, operators using conventional printing presses are forced to choose between expensive, high quality print runs using expensive inks or more cost effective, low quality print runs using less expensive inks.